Esophageal cancer is the eighth most common cancer worldwide, and the sixth most common cause of death among cancers [1]. Of the two types of esophageal cancer, adenocarcinoma (EAC) and squamous cell carcinoma (ESCC), 90% are ESCC, with rates increasing significantly in developing countries [2]. Patients suffering from ESCC have low, five-year survival rate, approximately 13% [3]. The prognosis of ESCC is often poor due to lack of effective treatment [4]. As a result of this limitation newer agents and novel approaches are imperative. Of particular interest is the chemotherapeutic application of curcumin, the major active ingredient of turmeric (Curcumin longa) [5-8]. Curcumin induces cell death in some cancers, such as gastric and colon cancers [9], human melanoma [10], and lung cancer [1,1] without major cytotoxic effects on healthy cells [12, 13]. Curcumin induces cell death through a variety of mechanisms by targeting pathways acting through a range of transcription factors, membrane receptors, kinases, and cytokines (reviewed by Anand et al. [14]). Therefore, curcumin has a potential treatment value for cancer either alone or in combination with other treatments, namely chemotherapy [15] and radiation treatments [16]. Although it is rapidly degraded and thus may have little effect outside of the digestive tract [17, 18], curcumin could be effective for treating ESCC because of its direct contact with epithelial cells lining the esophagus during ingestion. One possible reason for the poor prognosis of ESCC is the presence of cancer stem cells (CSCs) in the tumor [19,20]. It is believed that CSCs regenerate themselves and differentiate into non-CSCs that constitute most of the tumor volume [21-24]. Furthermore, CSCs tend to resist currently used cancer treatments, specifically chemotherapy and radiation therapy [25-27]. Therefore, the development of effective treatments for cancer should target this cell subpopulation [28]. Interestingly, curcumin with or without 5-fluorouracil and oxaliplatin significantly reduced the number of cells showing CSC-markers in a colon cancer cell line that had survived previous treatment with 5-fluorouracil [29].
Aldehyde dehydrogenase (ALDH) is highly expressed in CSCs, and thus can be used as a potential marker for identifying and isolating CSCs [30-32]. ALDH is a detoxifying enzyme responsible for the oxidation of both intracellular aldehydes such as ethanol and xenobiotic aldehydes such as cyclophosphamide [33, 34]. High ALDH expression could also indicate the aggressiveness, invasiveness, or metastatic capability associated with different cancers [26, 35]. ALDH1A1 is a member of the ALDH family that participates in alcohol metabolism and offers cellular protection against cytotoxic drugs [36]. Immunohistochemistry with a specific antibody has been used to identify human epithelial cells expressing ALDH1A1 [37]. Although ALDH1A1 has been identified in different type of cancer, this marker has not been evaluated in esophageal cancer.
Another marker for CSCs is CD44 (cluster of differentiation 44). CD44 is an integral cell membrane glycoprotein involved in cell-cell interaction [38]. It has been identified in many types of CSCs, including breast cancer cells [39], head and neck cancer [40], and gastric cancer [41]. High CD44 expression has also been shown to be associated with metastatic and invasive capabilities [42]. In this study, the effects of curcumin on six human ESCC lines were evaluated, and the lines were examined for the presence of CSCs using ALDHIAI and CD44 markers. In addition, the characteristics of curcumin-surviving cells and the original untreated cell lines were compared to determine whether CSCs could be targeted by curcumin treatment. By understanding the effects of curcumin on the stem cell subpopulation of tumors, the importance of its effects on cancer outcome could be recognized.
The ESSC and ESCC-S cells are a set of cell lines that can be used as a kit to determine the relative effectiveness of potential chemotherapeutic agents designed to inhibit neoplasm growth or metastasis. The kit will be particularly useful for testing chemotherapeutic agents that are intended to selectively target CSCs. The cell lines have been ranked according to their CSC content and according to their sensitivity to curcumin, a naturally occurring chemical that is reported to act selectively on cancer stem cells. All stages of drug development could rely on this kit for differential evaluation of drug efficacy.
In addition to the features mentioned above, objects and advantages of the present invention will be readily apparent upon a reading of the following description and through practice of the present invention.